U.S. patent number 10,446,702 [Application Number 15/887,995] was granted by the patent office on 2019-10-15 for photoelectric conversion material and solar cell using the same.
This patent grant is currently assigned to Panasonic Intellectual Property Management Co., Ltd.. The grantee listed for this patent is Panasonic Intellectual Property Management Co., Ltd.. Invention is credited to Hiroko Okumura.
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United States Patent |
10,446,702 |
Okumura |
October 15, 2019 |
Photoelectric conversion material and solar cell using the same
Abstract
A photoelectric conversion material includes a germanane
derivative having a composition represented by
Ge.sub.XM.sub.YH.sub.Z. M includes at least one of Ga and In.
X.gtoreq.Y, X.gtoreq.Z>0, and X+Y=1 are satisfied. A solar cell
includes: a first electrode having electrical conductivity; a
second electrode having electrical conductivity; and a
light-absorbing layer between the first electrode and the second
electrode, the light-absorbing layer converting incident light into
electric charge. The light-absorbing layer includes the
photoelectric conversion material above.
Inventors: |
Okumura; Hiroko (Osaka,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Intellectual Property Management Co., Ltd. |
Osaka |
N/A |
JP |
|
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Assignee: |
Panasonic Intellectual Property
Management Co., Ltd. (Osaka, JP)
|
Family
ID: |
63104829 |
Appl.
No.: |
15/887,995 |
Filed: |
February 3, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180233608 A1 |
Aug 16, 2018 |
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Foreign Application Priority Data
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Feb 15, 2017 [JP] |
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2017-025726 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
31/032 (20130101); H01L 31/036 (20130101); C07F
7/30 (20130101); Y02E 10/50 (20130101) |
Current International
Class: |
H01L
31/032 (20060101); C07F 7/30 (20060101); H01L
31/036 (20060101) |
Field of
Search: |
;136/242-265 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2755241 |
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Jul 2014 |
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EP |
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61-228613 |
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Oct 1986 |
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JP |
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2003-528796 |
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Sep 2003 |
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JP |
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2016-127267 |
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Jul 2016 |
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JP |
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2013/035686 |
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Mar 2013 |
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WO |
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Other References
Comedi, D., and I. Chambouleyron. "Dopant Impurity-Induced Defects
in p-Type Doped Hydrogenated Amorphous Germanium." Applied Physics
Letters, vol. 69, No. 12, 1996, pp. 1737-1739.,
doi:10.1063/1.118014. (Year: 1996). cited by examiner .
Comedi, D., et al. "Properties of Gallium-Doped Hydrogenated
Amorphous Germanium." Physical Review B, vol. 52, No. 7, 1995, pp.
4974-4985., doi:10.1103/physrevb.52.4974. (Year: 1995). cited by
examiner .
Fajardo, F., and I. Chambouleyron. "Structural and Optoelectronic
Properties of Indium-Doped a-Ge:H Thin Films." Physical Review B,
vol. 52, No. 7, 1995, pp. 4965-4973., doi:10.1103/physrevb.52.4965.
(Year: 1995). cited by examiner .
Abstract of Fajardo, F., et al. "Indium and Gallium p-Type Doping
of Hydrogenated Amorphous Germanium Thin Films." Applied Physics
Letters, vol. 64, No. 24, 1994, pp. 3273-3275.,
doi:10.1063/1.111307. (Year: 1994). cited by examiner .
Elisabeth Bianco et al., "Stability and Exfoliation of Germanane: A
Germanium Graphane Analogue", ACS Nano, vol. 7, No. 5, Mar. 19,
2013, pp. 4414-4421. cited by applicant .
Maxx Q. Arguilla et al., "Synthesis and Stability of
Two-Dimensional Ge/Sn Graphane Alloys", Chemistry of materials,
vol. 26, Nov. 12, 2014, pp. 6941-6946. cited by applicant .
Fan Fan, "Exfoliation and Stability Studies of Germanane and its
Derivatives", Undergraduate Research Thesis, The Ohio State
University, Nov. 2014. cited by applicant.
|
Primary Examiner: Mekhlin; Eli S
Assistant Examiner: Carlson; Kourtney R S
Attorney, Agent or Firm: Renner, Otto, Boisselle &
Sklar, LLP
Claims
What is claimed is:
1. A photoelectric conversion material comprising a germanane
derivative having a composition represented by
Ge.sub.XM.sub.YH.sub.Z, wherein M includes at least one of Ga and
In, and X.gtoreq.Y, X.gtoreq.Z>0, and X+Y=1 are satisfied.
2. The photoelectric conversion material according to claim 1,
wherein the germanane derivative has a crystal structure belonging
to space group P6.sub.3mc.
3. The photoelectric conversion material according to claim 1,
wherein Y is 0.005 or more and 0.227 or less.
4. The photoelectric conversion material according to claim 3,
wherein M is Ga, and Y is 0.005 or more and 0.067 or less.
5. The photoelectric conversion material according to claim 4,
wherein Y is 0.039 or more and 0.067 or less.
6. The photoelectric conversion material according to claim 3,
wherein M is In, and Y is 0.005 or more and 0.227 or less.
7. The photoelectric conversion material according to claim 6,
wherein Y is 0.034 or more and 0.227 or less.
8. The photoelectric conversion material according to claim 1,
wherein a bandgap of the germanane derivative is 1.22 eV or more
and 1.58 eV or less.
9. The photoelectric conversion material according to claim 8,
wherein the bandgap of the germanane derivative is 1.43 eV or more
and 1.58 eV or less.
10. A solar cell comprising: a first electrode having electrical
conductivity; a second electrode having electrical conductivity;
and a light-absorbing layer between the first electrode and the
second electrode, the light-absorbing layer converting incident
light into electric charge, wherein the light-absorbing layer
includes the photoelectric conversion material according to claim
1.
Description
BACKGROUND
1. Technical Field
The present disclosure relates to a photoelectric conversion
material and particularly to a light-absorbing or charge separation
material for solar cells. The present disclosure also relates to a
solar cell using the photoelectric conversion material.
2. Description of the Related Art
Graphene is a layered compound having SP.sup.2 hybridization, and
silicene and germanene are layered compounds having mixed
SP.sup.2-SP.sup.3 hybridization. These layered compounds have high
mobility and are semimetals with no bandgap. By hydrogenating
graphene, silicene, and germanene, graphane (CH), silicane (SiH),
and germanane (GeH), respectively, having SP.sup.3 hybridization
are obtained. These compounds have a bandgap. Among them, graphane
has the largest bandgap, and germanane has the smallest
bandgap.
FIG. 13A is an illustration showing the crystal structure of
germanane as viewed in the direction of its C axis, and FIG. 13B is
an illustration showing the crystal structure of germanane as
viewed in a direction perpendicular to the C axis.
Elisabeth Bianco et al., ACS Nano, March 2013, Vol. 7, No. 5, pp.
4414-4421 report that the bandgap of germanane is 1.59 eV. It is
also stated that the electron mobility in germanane is estimated to
be 18,195 cm.sup.2/(Vs).
Maxx Q. Arguilla et al., Chemistry of materials, November 2014,
Vol. 26, pp. 6941-6946 and Fan Fan, Exfoliation and Stability
Studies of Germanane and its Derivatives, Undergraduate Research
Thesis, The Ohio State University, November 2014 disclose germanane
derivatives obtained by partial replacement of germanium with other
elements.
SUMMARY
There is a need for further improvement in the performance of solar
cells.
One non-limiting and exemplary embodiment provides a photoelectric
conversion material including a germanane derivative and capable of
improving the performance of a solar cell.
In one general aspect, the techniques disclosed here feature a
photoelectric conversion material comprising a germanane derivative
having a composition represented by Ge.sub.XM.sub.YH.sub.Z, wherein
M includes at least one of Ga and In, and X.gtoreq.Y,
X.gtoreq.Z>0, and X+Y=1 are satisfied.
It should be noted that general or specific embodiments may be
implemented as an element, a device, a module, a system, an
integrated circuit, a method, or any selective combination
thereof.
Additional benefits and advantages of the disclosed embodiments
will become apparent from the specification and drawings. The
benefits and/or advantages may be individually obtained by the
various embodiments and features of the specification and drawings,
which need not all be provided in order to obtain one or more of
such benefits and/or advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a cross-sectional view schematically showing an example
of a solar cell;
FIG. 1B is a cross-sectional view schematically showing another
example of the solar cell;
FIG. 1C is a cross-sectional view schematically showing yet another
example of the solar cell;
FIG. 2A is a photograph showing a crystal of a compound
(Ge.sub.0.970Ga.sub.0.030H.sub.0.970) in Example 2;
FIG. 2B is a photograph showing a crystal of a compound
(Ge.sub.0.966In.sub.0.034H.sub.0.966) in Example 5;
FIG. 2C is a photograph showing a crystal of a compound (GeH) in
Comparative Example 1;
FIG. 3 is a graph showing X-ray diffraction patterns of compounds
in Examples 1 to 4 and Comparative Example 1;
FIG. 4 is a graph showing X-ray diffraction patterns of compounds
in Examples 5 and 6 and Comparative Example 1;
FIG. 5 is a graph showing X-ray diffraction patterns of compounds
(GeH) in Comparative Examples 1 and 2;
FIG. 6A is a graph showing the Ga doping concentration dependence
of the lattice constant "a" of a germanane derivative;
FIG. 6B is a graph showing the Ga doping concentration dependence
of the lattice constant "c" of the germanane derivative;
FIG. 7A is a graph showing the In doping concentration dependence
of the lattice constant "a" of a germanane derivative;
FIG. 7B is a graph showing the In doping concentration dependence
of the lattice constant "c" of the germanane derivative;
FIG. 8A is a graph showing the DRA spectrum of the compound
(Ge.sub.0.970Ga.sub.0.030H.sub.0.970) in Example 2;
FIG. 8B is a graph showing the DRA spectrum of the compound
(Ge.sub.0.961Ga.sub.0.039H.sub.0.961) in Example 3;
FIG. 8C is a graph showing the DRA spectrum of the compound
(Ge.sub.0.933Ga.sub.0.067H.sub.0.933) in Example 4;
FIG. 9A is a graph showing the DRA spectrum of the compound
(Ge.sub.0.966In.sub.0.034H.sub.0.966) in Example 5;
FIG. 9B is a graph showing the DRA spectrum of the compound
(Ge.sub.0.773In.sub.0.227H.sub.0.773) in Example 6;
FIG. 10A is a graph showing the DRA spectrum of the compound (GeH)
in Comparative Example 1;
FIG. 10B is a graph showing the DRA spectrum of the compound (GeH)
in Comparative Example 2;
FIG. 11A is a graph showing the Ga doping concentration dependence
of the bandgap of a germanane derivative;
FIG. 11B is a graph showing the In doping concentration dependence
of the bandgap of a germanane derivative;
FIG. 12A is a graph showing the FT-IR spectrum of the compound
(Ge.sub.0.970Ga.sub.0.030H.sub.0.970) in Example 2;
FIG. 12B is a graph showing the FT-IR spectrum of the compound
(Ge.sub.0.966In.sub.0.034H.sub.0.966) in Example 5;
FIG. 12C is a graph showing the FT-IR spectrum of the compound
(GeH) in Comparative Example 1;
FIG. 13A is a plan view showing the crystal structure of germanane
as viewed in the direction of its c axis; and
FIG. 13B is a cross-sectional view of the crystal structure of
germanane.
DETAILED DESCRIPTION
Underlying knowledge forming the basis of the present disclosure is
as follows.
As described above, germanane and its derivatives have high
electron mobility. The mobility .mu. of carriers (electrons in this
case) is one of the factors determining the diffusion length of the
carriers. As can be seen from the following formulas, as the
mobility .mu. increases, the diffusion length L of the carriers
increases.
.tau. ##EQU00001## .times..mu. ##EQU00001.2##
.mu..times..times..tau. ##EQU00001.3## D: diffusion constant,
.tau..sub.bulk: carrier lifetime, k: Boltzmann constant, T:
absolute temperature, q: electric charge, m*: effective mass,
.tau..sub.s: relaxation time
When high-mobility germanane or its derivative is used as a
photoelectric conversion material of a solar cell, the diffusion
length L of carriers generated by light absorption is large, and
therefore the carriers can easily reach an electrode without
electron-hole recombination. Since the amount of current that can
be outputted to the outside increases, the performance of the solar
cell is expected to be improved.
It is known that the performance of a photoelectric conversion
material for solar cells depends on its bandgap. The details are
described in William Shockley et al., Journal of Applied Physics,
March 1961, Vol. 32, No. 3, pp. 510-519. The limit of conversion
efficiency is known as the Shockley-Queisser limit. When the
bandgap is 1.4 eV, the theoretical conversion efficiency is
maximum. When the bandgap is larger than 1.4 eV, a high
open-circuit voltage is obtained, but the value of short-circuit
current decreases because the absorption wavelength decreases. When
the bandgap is less than 1.4 eV, the value of short-circuit current
increases because the absorption wavelength increases, but the
open-circuit voltage decreases.
As described above, the bandgap of germanane is 1.59 eV, which is
larger than 1.4 eV that gives the maximum theoretical efficiency.
There is therefore a need for a germanane derivative having a
bandgap closer to 1.4 eV. When such a germanane derivative is used
as a light-absorbing material for a solar cell, the solar cell
obtained can have higher conversion efficiency than conventional
solar cells.
The present inventors have found that, by doping germanane with
gallium (Ga) or indium (In), which is a group 13 element, to
partially replace Ge with Ga or In, the bandgap can be reduced
while a reduction in electron mobility is prevented.
By adjusting the bandgap to a value closer to 1.4 eV, a
photoelectric conversion material that can provide higher
conversion efficiency when used for a solar cell can be obtained.
Germanane has SP.sup.3 hybridization. However, by partially
replacing Ge with Ga or In, part of the SP.sup.3 hybridization
become SP.sup.2 hybridization, and the in-plane motion of electrons
is facilitated. Therefore, even when Ge is partially replaced with
Ga or In, the high electron mobility can be maintained. The high
electron mobility and the desired bandgap can thereby be achieved
simultaneously.
An outline of an aspect of the present disclosure is as
follows.
A photoelectric conversion material in the aspect of the present
disclosure comprises a germanane derivative having a composition
represented by Ge.sub.XM.sub.YH.sub.Z, wherein M includes at least
one of Ga and In, and X.gtoreq.Y, X.gtoreq.Z>0, and X+Y=1 are
satisfied.
The germanane derivative may have a crystal structure belonging to,
for example, space group P6.sub.3mc.
For example, Y may be 0.005 or more and 0.227 or less.
For example, M may be Ga, and Y may be 0.005 or more and 0.067 or
less.
For example, Y may be 0.039 or more and 0.067 or less.
For example, M may be In, and Y may be 0.005 or more and 0.227 or
less.
For example, Y may be 0.034 or more and 0.227 or less.
For example, a bandgap of the germanane derivative may be 1.22 eV
or more and 1.58 eV or less.
For example, a bandgap of the germanane derivative may be 1.43 eV
or more and 1.58 eV or less.
A solar cell in an aspect of the present disclosure comprises: a
first electrode having electrical conductivity; a second electrode
having electrical conductivity; and a light-absorbing layer between
the first electrode and the second electrode, the light-absorbing
layer converting incident light into electric charge, wherein the
light-absorbing layer includes the photoelectric conversion
material described above.
Embodiments
A photoelectric conversion material in an embodiment includes a
germanane derivative in which Ge in germanane is partially replaced
with at least one of Ga and In.
The germanane derivative in the present embodiment has a
composition represented by Ge.sub.XM.sub.YH.sub.Z (0<X, 0<Y,
and 0<Z). M includes at least one of Ga and In. The molar ratio
X of Ge (hereinafter referred to as the Ge ratio), the molar ratio
Y of M (hereinafter referred to as the M ratio), and the molar
ratio Z of H (hereinafter referred to as the H ratio) satisfy
relations represented by the following formulas (1) to (3). X+Y=1
(1) X.gtoreq.Y (2) X.gtoreq.Z (3) The following formula (4) may be
satisfied. 0<Z.ltoreq.1-Y (4) The germanane derivative may have
symmetry belonging to space group P6.sub.3mc.
The germanane derivative in the present embodiment can be obtained
by doping germanane with at least one of Ga and In at a ratio
(i.e., a molar ratio Y) of, for example, more than 0 mol % and 30
mol % or less.
The molar ratio Y of M in the germanane derivative may be, for
example, 0.005 or more and 0.227 or less. When M is Ga, i.e., when
the germanane derivative is Ge.sub.XGa.sub.YH.sub.Z, the molar
ratio Y may be, for example, 0.005 or more and 0.067 or less, or
may be 0.039 or more and 0.067 or less. When M is In, i.e., when
the germanane derivative is Ge.sub.XIn.sub.YH.sub.Z, the molar
ratio Y may be, for example, 0.005 or more and 0.227 or less, or
may be 0.034 or more and 0.227 or less. In the above cases, the
lower limit of the molar ratio Y is 0.005. However, when the molar
ratio Y is larger than 0, a certain effect can be obtained.
In the present embodiment, by partially replacing Ge in GeH with Ga
and/or In, the germanane derivative can have a smaller bandgap than
germanane while a reduction in electron mobility is prevented, as
described above. It is stated in Elisabeth Bianco et al., ACS Nano,
March 2013, Vol. 7, No. 5, pp. 4414-4421 that the bandgap of
germanane is 1.59 eV, although its production conditions are
different. In the present embodiment, by partially replacing Ge in
GeH with Ga and/or In, the germanane derivative obtained can have a
bandgap smaller than the bandgap of germanane, i.e., smaller than
1.59 eV described in Elisabeth Bianco et al., ACS Nano, March 2013,
Vol. 7, No. 5, pp. 4414-4421. The bandgap of the germanane
derivative in the present embodiment may be, for example, 1.58 eV
or less. The bandgap may be desirably 1.52 eV or less and more
desirably 1.50 eV or less. The lower limit of the bandgap may be,
for example, 1.22 eV or more and desirably 1.3 eV or more. Within
the above range, the bandgap is close to the ideal bandgap of 1.40
eV, and a solar cell having higher conversion efficiency than
conventional solar cells can be obtained. The bandgap can be
adjusted by changing the doping concentration of Ga or In or the
degree of vacuum during annealing.
The germanane derivative in the present embodiment can be
synthesized, for example, as follows. First, Ca, Ge, and Ga or In
are fired under prescribed conditions to obtain calcium germanide
(CaGe.sub.2) in which Ge is partially replaced with Ga or In. Next,
the obtained calcium germanide in a solid phase state is allowed to
react to replace calcium with hydrogen. In this method, germanane
(GeH) with Ge partially replaced with Ga or In is synthesized. A
specific synthesis method will be described in (Examples).
(Structure of Solar Cell)
The photoelectric conversion material in the present embodiment is
desirably applicable to solar cells.
FIGS. 1A to 1C are schematic cross-sectional views showing examples
of a solar cell 100 using the photoelectric conversion material in
the present embodiment.
In the solar cell 100 in FIG. 1A, a first electrode 103, a
light-absorbing layer 102, and a second electrode 104 are stacked
in this order on a substrate 101.
In each of the solar cells 100 in FIGS. 1B and 1C, a
light-absorbing layer 102 is disposed on a substrate 101, and a
first electrode 103 and a second electrode 104 are disposed on the
light-absorbing layer 102 with a prescribed space therebetween. In
the germanane derivative, the mobility is particularly high in
in-plane directions. In the structures shown in FIGS. 1B and 1C,
the high mobility in the germanane derivative in the in-plane
directions can be effectively utilized.
The light-absorbing layer 102 converts incident light to electric
charge. The light-absorbing layer 102 includes the photoelectric
conversion material in the present embodiment. The photoelectric
conversion material includes the germanane derivative described
above. The light-absorbing layer 102 may be obtained by slicing the
germanane derivative (a laminar crystal) synthesized by a method
described later into a prescribed size. Alternatively, the
light-absorbing layer 102 may be formed by growing the germanane
derivative on a surface of the substrate or an electrode.
In each solar cell 100, when the light-absorbing layer 102 is
irradiated with light from the outside, the light is absorbed, and
electrons and holes are generated. The electrons generated in the
light-absorbing layer 102 are outputted to the outside through the
first electrode 103. The holes generated in the light-absorbing
layer 102 are outputted to the outside through the second electrode
104.
The substrate 101 plays a role in physically holding the
light-absorbing layer 102, the first electrode 103, and the second
electrode 104. For example, a transparent material or a
non-transparent material may be used for the substrate 101.
Examples of the transparent material include glass and
light-transmitting plastics. Examples of the non-transparent
material include metals, ceramics, and non-light-transmitting
plastics. When a transparent material is used for the substrate
101, the light-absorbing layer 102 may be irradiated with sunlight
passing through the substrate 101 to generate electric power.
In the structure in FIG. 1A, when one or both of the first
electrode 103 and the second electrode 104 have sufficiently high
strength, the substrate 101 may be omitted. The substrate 101 is
disposed in contact with the first electrode 103. However, the
substrate 101 may be disposed in contact with the second electrode
104.
A conductive material may be used for the first electrode 103 and
the second electrode 104. Examples of the conductive material
include metals, transparent metal oxide materials, and carbon
materials. Examples of the metal material include gold, silver,
copper, platinum, aluminum, titanium, nickel, tin, zinc, and
chromium. Examples of the transparent metal oxide material include
indium-tin complex oxide, antimony-doped tin oxide, fluorine-doped
tin oxide, and zinc oxide doped with boron, aluminum, gallium, or
indium. Examples of the carbon material include graphene, carbon
nanotubes, and graphite.
In particular, in the structure in FIG. 1A, it is desirable that
one or both of the first electrode 103 and the second electrode 104
have light transmittance in the ultraviolet to near infrared range.
When a non-transparent material is used for one of the first
electrode 103 and the second electrode 104, light transparency can
be imparted by providing a pattern for light transmission. Examples
of the pattern include a grating pattern, a line pattern, and a
wavy line pattern.
In the structure in FIG. 1A, when the first electrode 103 and the
second electrode 104 have light transparency, it is desirable that
the light transmittance of the electrodes 103 and 104 is high. The
light transmittance is, for example, 50% or higher and desirably
80% or higher. Desirably, the wavelength range of light
transmitting through the electrodes 103 and 104 is wider than the
absorption wavelength range of the light-absorbing material
included in the light-absorbing layer 102.
Although not illustrated, an electron transport layer may be
disposed between the light-absorbing layer 102 and the first
electrode 103. By disposing the electron transport layer, the
electron extraction efficiency from the first electrode 103 can be
improved. The electron transport layer is typically formed from a
semiconductor material.
Examples of the semiconductor material used for the electron
transport layer include metal oxide materials and organic n-type
semiconductor materials. Examples of the metal oxide materials
include titanium oxide, tin oxide, zinc oxide, and indium oxide.
Examples of the organic n-type semiconductor materials include
imide compounds, quinone compounds, fullerenes, and their
derivatives.
Although not illustrated, a hole transport layer may be disposed
between the light-absorbing layer 102 and the second electrode 104.
By disposing the hole transport layer, the hole extraction
efficiency from the second electrode 104 can be improved. The hole
transport layer is typically formed from a semiconductor
material.
Examples of the semiconductor material used for the hole transport
layer include inorganic p-type semiconductor materials and organic
p-type semiconductor materials. Examples of the inorganic p-type
semiconductor materials include CuO, Cu.sub.2O, CuSCN, molybdenum
oxide, and nickel oxide. Examples of the organic p-type
semiconductor materials include phenylamines having a tertiary
amine in their structure, triphenylamine derivatives, and PEDOT
compounds having a thiophene structure.
Examples
A plurality of germanane derivatives containing different dopants
at different concentrations were produced and subjected to
analysis. In Examples 1 to 4, germanane derivatives containing Ga
were produced. In Examples 5 and 6, germanane derivatives
containing In were produced. In Comparative Examples 1 and 2,
germanane (GeH) was produced.
Methods for Producing Compounds in Examples and Comparative
Examples
Examples 1 to 4
First, a quartz ampule having a closed end was loaded with Ca, Ge,
and Ga in a nitrogen atmosphere. The purity of Ca used was 99%, the
purity of Ge used was 99.999% or higher, and the purity of Ga used
was 99.99%. The materials to be loaded into the quartz ampule were
sufficiently mixed in advance in a mortar under a nitrogen
atmosphere. The ratio of the number of moles of Ga to ((the number
of moles of Ge)+(the number of moles of Ga)) (this ratio is also
referred to as the initial prepared ratio) in each Example is shown
in Table 1. The ratio of the total number of moles of Ge and Ga
used as raw materials of a germanane derivative to the number of
moles of Ca was 2.
Next, a rotary pump was used to evacuate the quartz ampule to
3.0.times.10.sup.-2 Pa, and then an opening of the quartz ampule in
the evacuated state was sealed using a burner (oxygen-hydrogen
torch).
Next, the mixture was annealed in an electric furnace at
1,000.degree. C. for 18 hours. Then the resulting mixture was
cooled to room temperature over three days. In this case, the
mixture was cooled to 386.degree. C. in 48 hours in the electric
furnace and was then left to cool naturally.
Next, the fired material was immersed in 5 mol/L HCl (aq), i.e., an
aqueous HCl solution, for 1 day and washed with ion exchanged
water. In this case, a germanane derivative (solid) was separated
by spontaneous sedimentation. The separated germanane derivative
was further washed with ethanol and dried sufficiently in a vacuum
drying oven. Compounds in Examples 1 to 4 were obtained in the
method described above.
Examples 5 and 6
Compounds in Examples 5 and 6 were produced using the same method
as in Example 1 except that In with a purity of 99.99% was used as
a raw material instead of Ga. The molar ratio of In (also referred
to as the initial prepared ratio) in each Example is shown in Table
1.
Comparative Examples 1 and 2
Only Ca and Ge were placed in a quartz ampule and fired, washed,
and dried using the same method as in Example 1 to thereby produce
a compound (GeH) in Comparative Example 1. A compound (GeH) in
Comparative Example 2 was produced by the same method as in
Comparative Example 1 except that the quartz ampule was evacuated
during sintering to 3.0.times.10.sup.-3 Pa using a turbo-molecular
pump.
Photographs of the compounds in Examples 2 and 5 and Comparative
Example 1 are shown in FIGS. 2A, 2B, and 2C, respectively.
<Measurement of Doping Concentration: ICP-AES
Measurement>
The doping concentrations of Ga or In in the compounds in Examples
1 to 6 were examined.
First, the compound in each Example was subjected to pretreatment.
Specifically, the compound was dissolved in sulfuric acid and
nitric acid and diluted with pure water to obtain a solution. For
each of the solutions obtained, the doping concentration of Ga or
In was measured by ICP-AES (inductively-coupled plasma atomic
emission spectrometry). The CIROS-120 manufactured by Spectro was
used for the measurement. The measurement results and the
composition of each compound are shown in Table 1.
<Lattice Constants: Powder XRD Measurement>
The lattice constants of each of the compounds in the Examples and
the Comparative Examples were determined by powder XRD (X-ray
diffraction) measurement.
The RINT 2000 manufactured by Rigaku Corporation was used for the
XRD measurement, and a vertical goniometer was used as the optical
system. The measurement angle range was 10.degree. to 80.degree.,
and the scanning speed was 2.3.degree./m in.
FIG. 3 is a graph showing the X-ray diffraction patterns of the
compounds in Examples 1 to 4 and Comparative Example 1. FIG. 4 is a
graph showing the X-ray diffraction patterns of the compounds in
Examples 5 and 6 and Comparative Example 1. FIG. 5 is a graph
showing the X-ray diffraction patterns of the compounds in
Comparative Examples 1 and 2. For comparison, the X-ray diffraction
pattern of germanium is also shown in FIGS. 3 and 4. The
measurement was performed in the range of 10.degree. to 80.degree.,
but the X-ray diffraction patterns in the range of 10.degree. to
40.degree. are shown in FIGS. 3 to 5.
As can be seen from FIGS. 3 to 5, Ge peaks originated from the
precursor were found in the range of 25.degree. to 28.degree. in
some of the X-ray diffraction patterns in the Examples and the
Comparative Examples. The lattice constants of each of the
compounds in the Examples and the Comparative Examples were
computed using the Cohen method from (002), (100), and (011) peaks
on the low-angle side in the measurement results. On the low-angle
side, measurement error is considered to be relatively small. The
results are shown in Table 1.
The relation between the doping concentration of Ga or In and the
lattice constants was examined. FIG. 6A is a graph showing the
relation between the doping concentration of Ga and the lattice
constant "a" in the a-axis direction, and FIG. 6B is a graph
showing the relation between the doping concentration of Ga and the
lattice constant "c" in the c-axis direction. FIG. 7A is a graph
showing the relation between the doping concentration of In and the
lattice constant "a" in the a-axis direction, and FIG. 7B is a
graph showing the relation between the doping concentration of In
and the lattice constant "c" in the c-axis direction.
As can be seen from these results, the doping with Ga or In can
increase the lattice constants, and the lattice constants can be
larger than those of GeH (Comparative Examples 1 and 2). As can be
seen, the lattice constants vary depending on the doping
concentration. For example, as the doping concentration of Ga
increases, the lattice constant "a" in the a-axis direction and the
lattice constant "c" in the c-axis direction increase. In the X-ray
diffraction patterns shown in FIGS. 3 and 4, the positions of the
peaks of the (002), (100), and (011) planes of the compounds in
Examples 1 to 6 correspond to the crystal structure of space group
P6.sub.3mc. Therefore, the compounds in Examples 1 to 6 have a
crystal structure belonging to space group P6.sub.3mc.
<Bandgap: Diffuse Reflectance Absorption Measurement>
The bandgap of each of the compounds in the Examples and the
Comparative Examples was determined by diffuse reflectance
absorption measurement.
The UV-3600Plus manufactured by Shimadzu Corporation was used for
the diffuse reflectance absorption measurement, and the ISR-603 was
used as the integrating sphere. The measurement was performed in a
spectral measurement wavelength range of 400 nm to 1,300 nm at a
scanning speed of 200 nm/min, using a sampling width of 1.0 nm and
a slit width of 32 nm. The incident angle was 0.degree., and
specular reflection was not included. Barium sulfate was used as a
standard sample, and a measurement range of 5 mm square was
used.
The diffuse reflectance spectrum obtained by the measurement was
subjected to Kubelka-Munk transformation to convert it to an
absorption spectrum.
FIGS. 8A to 8C are graphs showing the DRA spectra in Examples 2 to
4. FIGS. 9A and 9B are graphs showing the DRA spectra in Examples 5
and 6. FIGS. 10A and 10B are graphs showing the DRA spectra in
Comparative Examples 1 and 2.
Next, the bandgaps in the Examples and the Comparative Examples
were computed from the absorption spectra obtained. The results are
shown in Table 1.
The relation between the doping concentration of Ga or In and the
bandgap was examined. FIG. 11A shows the relation between the
doping concentration of Ga and the bandgap of the germanane
derivative, and FIG. 11B shows the relation between the doping
concentration of In and the bandgap of the germanane
derivative.
As can be seen from these results, the doping with Ga or In can
reduce the bandgap, and the bandgap can be lower than that of
germanane (Comparative Example 1), irrespective of the doping
concentration. Although not illustrated, the effect of reducing the
bandgap may be obtained even when the doping amount of Ga or In is
very small (e.g., about 0.005). In particular, when the doping
concentration of Ga is 0.039 or more and 0.067 or less, the bandgap
can be further reduced and can be closer to 1.40 eV.
As can be seen from the results in Comparative Examples 1 and 2,
the bandgap tends to decrease as the degree of vacuum increases
during sintering. Therefore, by doping with Ga or In and increasing
the degree of vacuum during sintering, a lower bandgap may be
obtained.
<FT-IR Measurement>
Next, Fourier transform infrared spectrophotometry (FT-IR) was used
to check whether Ge--H bonds were formed in the compounds of
Examples 2 and 5 and Comparative Example 1.
The compound in each of Examples 2 and 5 and Comparative Example 1
was subjected to pretreatment. Specifically, the compound was mixed
with KBr to produce pellets for analysis. The analyzer used was the
iS10 manufactured by Thermo Fisher Scientific, and the detector
used was the DLaTGS/KBr. In the measurement, the cumulated number
was 128, and the resolution was 4 cm.sup.-1.
FIGS. 12A to 12C are graphs showing the absorption spectra of the
compounds in Examples 2 and 5 and Comparative Example 1.
As can be seen from the analysis results, a Ge--H stretching mode
peak was detected at 2006 cm.sup.-1 to 2003 cm.sup.-1, and Ge--H
wagging mode peaks were detected at 829 cm.sup.-1 to 826 cm.sup.-1,
772 cm.sup.-1 to 760 cm.sup.-1, 578 cm.sup.-1 to 569 cm.sup.-1, and
482 cm.sup.-1 to 480 cm.sup.-1. The peaks of a Ge--H hydrate or
Ge--H hydrates were detected at 3,418 cm.sup.-1 to 3,386 cm.sup.-1
and 1,639 cm.sup.-1 to 1,620 cm.sup.-1. The peaks of an aliphatic
hydrocarbon, which is an impurity mixed during the material
synthesis process, were detected at 2,924 cm.sup.-1 and 2,853
cm.sup.-1. The formation of Ge--H bonds was observed also in the Ga
or In doped compounds in Examples 2 and 5.
TABLE-US-00001 TABLE 1 Initial Doping prepared ratio concentration
of In or Ge of Ga or In Lattice constant Bandgap (mol %) (mol %)
Composition a (.ANG.) c (.ANG.) (eV) Example 1 Ga: 2.16 Ga: 1.82
Ge.sub.0.982Ga.sub.0.018H.sub.0.982 3.860 10.681 -- Example 2 Ga:
3.75 Ga: 2.95 Ge.sub.0.970Ga.sub.0.030H.sub.0.970 3.936 10.944 1.60
Example 3 Ga: 5.97 Ga: 3.92 Ge.sub.0.961Ga.sub.0.039H.sub.0.961
3.938 11.229 1.43 Example 4 Ga: 10.03 Ga: 6.70
Ge.sub.0.933Ga.sub.0.067H.sub.0.933 3.964 11.282 1.51 Example 5 In:
3.00 In: 3.44 Ge.sub.0.966Ga.sub.0.034H.sub.0.966 3.902 11.235 1.52
Example 6 In: 9.87 In: 22.7 Ge.sub.0.773Ga.sub.0.227H.sub.0.773
3.873 11.279 1.52 Comparative -- -- GeH 3.860 11.074 1.74 Example 1
Comparative -- -- GeH 3.886 11.127 1.55 Example 2
The composition of the germanane derivative in the present
embodiment is not limited to the compositions in Examples 1 to 6.
Although not shown in the Examples,
Ge.sub.XGa.sub.Y1In.sub.Y2H.sub.Z (X+Y1+Y2=1) may be synthesized by
doping with both Ge and In. Also in this case, the effects in the
above Examples are obtained. In Table 1, the compositions of the
compounds in Examples 1 to 6 are based on the assumption that Ge is
fully hydrogenated. However, Ge is not necessarily fully
hydrogenated. Since the influence of the hydrogenation ratio of Ge
is small, the same effects are obtained. Specifically, in
Ge.sub.X(Ga or In).sub.YH.sub.Z, the relation X.gtoreq.Z>0 may
hold.
The photoelectric conversion material of the present disclosure is
useful as a material for a light-absorbing layer of a solar cell.
The photoelectric conversion material is applicable to devices for
conversion of light to electricity such as optical sensors and
light-emitting devices.
* * * * *